Milk production has evolved over the course of evolution. As such, milk has developed as the most efficient, effective and adaptable nutrient provision that has risen among vertebrates. Milk has played an important role in the worldwide, adaptive radiation of mammals. It has conferred a primary advantage on mammals; that is, the ability to reproduce and nourish their young in any environment that supports the health and wellbeing of the adult. Milk from species such as the marsupial and the monotreme, whose young rely solely on milk at a time when regulated development is paramount, provide useful insight into the function of milk.